As regards a method of working a cold-rolled steel sheet to form a battery case, a method appropriately combining deep drawing with ironing is used. Examples of the method include a DI process in which a steel sheet is deep-drawn to form a drawn cup and ironing is then performed; stretch-draw forming in which a steel sheet is deep-drawn to form a drawn cup, stretching and bending-unbending are then performed, and ironing is further performed, as needed; and a multi-stage drawing process in which drawing is performed in several stages, and ironing is then performed.
In working a battery case, it is desired to suppress the generation of ears (called “earing”), more specifically, to prevent the height of the can after the working from being uneven in the circumferential direction thereof. It is generally known that the height of such an ear significantly correlates with planar anisotropy Δr of the r value (Lankford value) of a steel sheet for a battery case, and that as Δr approaches zero, the height of the ear decreases. Accordingly, to suppress earing, desirably, Δr of the steel sheet for a battery case is controlled to be close to zero.
To reduce Δr of a cold-rolled steel sheet, means for increasing the rolling reduction in cold rolling is usually employed. However, at a-high rolling reduction, the dependency of Δr on the rolling reduction increases, and thus, a variation in Δr is easily generated and an increase in the rolling load due to the high rolling reduction may be caused. Therefore, it is desired that a cold-rolled steel sheet be produced at a rolling reduction of at least 90% or less, and more preferably, 85% or less.
To prevent degradation of a can shape due to wrinkles called “stretcher strain” generated during deep drawing, it is desired that a steel sheet for a battery case have excellent strain aging resistance. In addition, to suppress surface roughening during working, it is also desired that a steel sheet for a battery case have fine crystal grains.
As such a steel sheet for a battery case, hitherto, interstitial free steels (IF steels), which are suitable for deep drawing and to which Nb and/or Ti is added, have been studied. For example, Japanese Unexamined Patent Application Publication No. 10-81919 discloses a method of producing a steel sheet for a two-piece can having an excellent earing property and surface roughening resistance. In the method, a steel slab containing at least one type of element selected from, by mass percent, 0.0005% to 0.0150% of C, 0.10% or less of Si, 0.1% to 0.6% of Mn, 0.02% or less of P, 0.02% or less of S, 0.015% to 0.15% of Al, 0.02% or less of N, 0.020% or less of Nb, 0.020% or less of Ti, and 0.0001% to 0.0030% of B, and the balance composed of Fe and inevitable impurities is hot-rolled, cold-rolled, and annealed to form recrystallized grains having an ASTM grain size number of 10 or more and a crystal grain axial ratio of 1.2 or less. Subsequently, a secondary cold rolling is performed at a rolling reduction in the range of 0.5% to 40%.
In addition, Japanese Unexamined Patent Application Publication No. 9-310150 discloses a steel sheet for a can having excellent workability, earing property, and surface roughening resistance. The steel sheet for a can contains at least one type of element selected from, by mass percent, 0.0005% to 0.015.0% of C, 0.10% or less of Si, 0.1% to 0.6% of Mn, 0.02% or less of P, 0.02% or less of S, 0.015% to 0.15% of Al, 0.02% or less of N, 0.020% or less of Nb, and 0.020% or less of Ti, and the balance composed of Fe and inevitable impurities, wherein crystal grains in a surface layer region from the surface of the steel sheet to 1/10 of the sheet thickness are composed of fine equiaxed crystal grain structure having an ASTM grain size number of 10 or more and a crystal grain axial ratio of 1.5 or less, and crystal grains in the inner layer of the steel sheet except for this surface layer are composed of coarse equiaxed crystal grain structure having an ASTM grain size number of 9 or less and a crystal grain axial ratio of 1.5 or less.
Furthermore, Japanese Unexamined Patent Application Publication No. 63-310924 discloses a method of producing an ultra-thin steel sheet having small planar anisotropy. In this method, a steel sheet having, a composition containing, by mass percent, 0.004% or less of C, 0.1% or less of Si, 0.5% or less of Mn, 0.025% or less of P, 0.025% or less of S, 0.006% or less of N, 0.001% to 0.100 of Al, 0.01% to 0.10% of Ti wherein the relationship Ti≧{(48/12)C+(48/14)N} is satisfied, 0.003% to 0.03% of Nb, and 0.0001% to 0.0010% of B, and the balance substantially composed of Fe except for inevitable impurities is hot-rolled under the conditions of a hot-rolling finishing temperature in the range of 850° C. to 900° C. and a winding temperature in the range of 300° C. to 600° C., cold-rolled, and then undergoes continuous annealing, followed by skin-pass rolling to reduce the thickness thereof in the range of 0.15 to 0.60 mm, wherein the cold-rolling reduction is controlled to be in the range of 85% to 95%, and the continuous annealing temperature is controlled to be in the range of 650° C. to 750° C.
However, in the steel sheets described in Japanese Unexamined Patent Application Publication Nos. 10-81919, 9-310150, and 63-310924, when the rolling reduction of cold rolling is 85% or less, which is a preferable value, the average grain size of a ferrite structure does not always stably become small (specifically, 12.0 μm or less), and Δr that is sufficiently close to zero (specifically, −0.20≦Δr≦0.20) is not always obtained.
It could therefore be helpful to provide a cold-rolled steel sheet in which even when the rolling reduction of cold rolling is, 85% or less, the average grain size of a ferrite structure can be reliably controlled to be 12.0 μm or less and the relationship −0.20≦Δr≦0.20 can be reliably satisfied and which has an excellent earing property, and a method of producing the same.